1. Field of the Invention
This invention relates to a control system for a machine tool in which an optimum feed rate of cutting of the machine tool is calculated at the outset, a tool path as well as an optimum feed rate of cutting calculated is directly output to a driving unit of the machine tool, and in which a work and the tool are relatively moved along the tool path at an optimum cutting feed rate at each part of the tool path.
2. Description of Related Art
A conventional machine tool includes a CNC controller that controls the driving of the driving motor based on NC (numerical control) data composed of NC programs termed ‘G code’. If, in such conventional machine tool, a free curved surface, for example, is to be cut, an operating command is issued at each of a number of extremely short line segments, as shown in Patent publication 1. The CNC controller is instructed by NC data so as to render the cutting feed rate constant. On receiving the NC data, the CNC controller of the conventional machine tool actuates a driving motor, via a motor amplifier, in accordance with input NC data.
However, in the CNC controller of a conventional machine tool, the G code is pre-read, at the time of the machining operation, so that the cutting feed rate is slowed down from the command value in such a manner that the values of the acceleration as well as those of the speed, allocated to the respective driving shafts, will not exceed respective marginal values thereof. The reason may be such that the CNC controller of a conventional machine tool uses an interpreter system in which an input NC program is sequentially analyzed and executed. On the other hand, even though the limits of the acceleration and the speed of the driving unit that actuates the respective driving shafts may be known beforehand, there lack data on the mass weight or the inertial force of a moving object, such as a work, during the machining operations. It is thus not possible to calculate the limit of the torque generated with acceleration beforehand.
Thus, in the conventional machine tool, the cutting feed rate is dropped by a value more than is necessary than the command value.
When e.g., a corner of a work being machined is cut, the acceleration will theoretically become infinitely large unless the work is brought to a standstill. For this reason, a CNC controller of a conventional machine tool provides for a cutting mode of starting the operation of the next driving shaft before the outstanding operation (mode G64) is brought to a standstill at the corner of the work. That is, the corner of the work is rounded in a compromising fashion in order to raise the speed of the operations.
In such cutting mode, an R not inherently present in an engineering drawing is formed only in a compromising fashion by the CNC controller in order to speed up the operation. Hence, the finishing shape of the work tends to deviate from what has been intended by the designer. In addition, since the R produced at the corner of the work is set in a sloppy manner, there persists a problem that the final shape may not be surmised until the time the work has ultimately been cut.